I'm certain a high percentage of the people on these Forums have been inside of caves or mines before, so I don't want this to sound in any way patronizing (or insulting, etc):
I've been in several caves myself, and had a friend who was a professional caver/writer on caving (he was a little, hairy, friendly (and crazy), bearded man who died from complications of alchoholism, so as close to a dwarf as I've ever personally known. His name was Hawkeye Hook. He was a good guy--a good friend to my dad, a good friend to me, and I wish he was still around. He could have brought a lot to the DF table.).
For those (few?) of you who haven't been inside of them before, cave environments *can* be extremely bizarre, and extremely dangerous, at times, and depending on the cave system.
Just being underground, like in a mine (I've been in coal mines before, we have lots of them in Pennsylvania) can still be weird, uncomfortable, and dangerous.
I'm the opposite of a claustrophobe-I'd rather be in a small, confined space, than a large one, infact I'm slightly agoraphobic-but there's a certain feeling of oppression, to being under a hundred feet or more of earth, that I think probably everyone feels-even a dwarf?-if you're not used to it, even if the place you're at isn't in any way particularly dangerous.
As far as the current situation in the game is concerned, it seems-to me, anyway-to resemble a vitamin deficiency, or maybe just good old hyperphotosensitivity.
There's something to be said for actually *adapting* to live in such an environment: Having the skills to move around safely, to find food and water, and to avoid dangers, knowing the environment well enough to adapt it to tactical combat-and knowing it well enough not to cause long-term harm to it, by your presence-and using all of your senses, but not relying on any one of them, to find your way around.
There's a lot to learn. Maybe even enough for a "caving" skill, someday?
So, in my opinion, cave adapatation should bring with it a movement bonus, a combat bonus, and a detection/perception bonus. That just makes sense to me.
Agility, I think, might go up a little faster, too, considering the terrain one sometimes has to travel over, and the methods for doing so.
I also think that some critters could/should come with more enhanced versions of "cave adaptation". With enhanced benefits, but also enhanced detrimental harm, if they venture outside.
To give you an example: Dwarfs might be at a 1, while GCSpiders might be at a 5? Cave trolls or mole men might fall between those two extremes?